Numerous known devices for transporting powder operate according to the venturi principle, in which the powder is carried along by a gas stream in a nozzle. Such devices are simple in construction but they have three serious disadvantages:
First, the powder density that can be achieved in the gas stream is very low and the powder is transported by air-borne entrainment, i.e. the air velocity must be greater than the suspension velocity. Second, the consistency of the quantity of powder transported is totally inadequate. Moreover, the quantity of powder is difficult to regulate. These disadvantages are especially serious when such pumps based on the venturi principle are employed to transport powder paints, since the resulting coatings demonstrate substantial fluctuations in film thickness and visual qualities.
Consequently, in the past, solutions have been sought which do not operate on the venturi principle.
A device is known from EP 1 106 547 A1 in which powder is transported pneumatically into what is termed a metering chamber. This metering chamber is connected to a suction line. The metering chamber is further connected to a pressure line, through which the powder is transported from the metering chamber into a discharge line.
In order to generate negative pressure in the suction line, this device requires an external device outside the metering chamber to generate negative pressure, for example, a vacuum pump. A control mechanism must be located between the pump and the metering chamber by which the passage for the gas can be closed off and opened. So that the powder flowing into the metering chamber cannot enter the suction line, the line is separated from the metering chamber by a gas permeable diaphragm. Depending on the makeup of the powder transported by this device, the diaphragm tends to become blocked or clogged, which naturally has a deleterious effect on the smooth operation of the device.
A second diaphragm pump for transporting powders is known from EP 0 124 933. It describes a pump having a piston which is moved up and down in a transfer chamber. The piston generates a negative pressure in the supply line on its upward path and sucks the powder out of the storage tank. Afterwards the powder is compressed in the transfer chamber by a downward movement of the piston. After the piston has reached bottom dead center, the discharge line is opened and the compressed powder is transported by means of compressed air to the application station.
In order to generate negative pressure, the piston must be sealed by a gasket, which leads to extreme wear and contamination of the moving parts. This pump generates a very inconsistent powder/air volume flow.
Furthermore, easily wettable powders, such as curable powder paints for example, easily cause blockages of the transfer chamber because of the compression prior to being transported.
This is probably the reason why this type of construction has not been accepted for the transportation of powder paints.
According to U.S. Pat. No. 3,391,963, blockage by powder of a diaphragm in a transportation device is to be prevented by moving the diaphragm back and forth by means of a piston. The diaphragm undulates, and the powder adhering thereto can be shaken off. This device operates without the provision of compressed air to the transfer chamber.
This device is expensive and susceptible to wear, since it requires a semi-permeable diaphragm. Additionally, the major part of the powder adhering is removed by the mechanical movement of the diaphragm. However, small amounts of powder remain on the surface of the diaphragm, so that clogging can be observed after an extended period of operation.
A device for pneumatically transporting bulk material is further known from DE 10 87 520 B which operates in accordance with the known dual-piston principle.
Porous inserts are similarly used with the device, separating the transport material from the operating elements of the piston system. This prevents the transport material from coming into contact with the operating elements. According to the teaching of this publication, woven materials, ceramic stone material or sintered metal can be used as porous inserts.
The critical disadvantage of this device is that when transporting fine powders, the inserts can quickly become gummed or clogged. Excessive susceptibility to wear and extremely abbreviated maintenance intervals are the consequence. Consequently, this device is also less suitable for transporting curable and fine powders, such as powder paints for example.